1. Field of the Invention
The present invention concerns a pipe of relatively small outer diameter such as less than about 20 mm disposed as a fluid transportation pathway to automobiles or like outer various machineries and, more in particular, it relates to a heat and corrosion resistant steel pipe having multi-layered coating with a protection layer capable of withstanding severe corrosive circumstance.
2. Description of the Prior Art
As pipeways used for automobiles or other machineries, those products having Zn-plating layer formed to the outer surface of a steel pipe and applied with chromate treatment to the Zn-plating layer have generally been used.
However, working conditions are severe, for example, in braking or fuel pipeways of automobiles and there have been demands for heat resistance under high temperature circumstance such as at the periphery of engines, resistance to external mechanical forces such as scratch resistance and impact shock resistance in underfloor pipeways that easily suffer from external injuries as well as higher corrosion resistance. For coping with such demands, those having a further plastic protection coating formed on the Zn-plating layer have also been used and Japanese Patent Publication Nos. Sho 57-60434 and Sho 61-23271 propose a method of applying a Zn-plating by means of electrolytic process to the surface of a metal object, forming a relatively thick special chromate layer to the Zn layer by using a chromic acid solution containing formic acid and then forming a fluoro resin layer on the chromate layer.
However, in the protection layer formed by the above-mentioned method, it is necessary to increase the Zn-plating layer as greater than 25 um in order to improve the corrosion resistance of the metal plating layer, which brings about a problem that the productivity is remarkably worsened Further, for attaining a tight bonding with the fluoro resin, it is necessary to form the chromate layer by using a chromic acid solution at relatively high concentration containing formic acid, which results in a problem that a lot of chemicals for the treating solution is consumed and the processing cost is increased for the solution wastes containing noxious hexavalent chromium.
The object of the present invention is to overcome the foregoing problems and to obtain a heat and corrosion resistant steel pipe having multi-layered coating which is durable under severe corrosive circumstance including external mechanical forces by improving the heat and corrosion resistance of a metal plating layer in a corrosion resistant protection layer comprising a metal plating layer and a resin layer laminated to each other and applied to the surface of the steel pipe and forming a resin layer with a tight adherance to the metal plating layer.
The present inventors have made an earnest study for overcoming the foregoing problems and attaining the foregoing object and, as a result, have accomplished the present invention based on the finding that the object can be attained by forming a multi-layered coating with an epoxy resin being intervened as an intermediate layer upon forming a fluoro resin layer on a metal plating layer forming the metal plating layer by laminating a Ni-plating layer of a specific thickness and a Znxe2x80x94Ni alloy plating layer. That is, the first embodiment of the present invention is a heat and corrosion resistant steel pipe having multi-layered coating comprising a steel pipe, a Znxe2x80x94Ni alloy plating layer deposited to the outer surface of the steel pipe and a polyvinyl fluoride layer formed on the Znxe2x80x94Ni alloy plating layer with an epoxy resin being intervened as an intermediate layer.
The second embodiment of the present invention is a heat and corrosion resistant steel pipe having multi-layered coating comprising a steel pipe, a Znxe2x80x94Ni alloy plating layer deposited on the outer surface of the steel pipe, a chromate layer formed on the Znxe2x80x94Ni alloy plating layer and a polyvinyl fluoride layer formed on the chromate layer with an epoxy resin being intervened as an intermediate layer.
Further, in the third embodiment of the present invention, a Ni-plating layer of 0.2-10 um thickness is present between the outer surface of a steel pipe and a Znxe2x80x94Ni alloy plating layer and a chromate layer is formed on the Znxe2x80x94Ni alloy plating layer with the Ni-plating layer being intervened.